Method and apparatus for desolvating ions for introduction into a faims analyzer region

ABSTRACT

Disclosed are an apparatus and a method for separating ions produced at an electrospray ionization source, based upon the high field mobility properties of the ions. An apparatus according to the instant invention includes a high field asymmetric waveform ion mobility spectrometer (FAIMS) having an analyzer region defined by a space between an inner electrode ( 32 ) and an outer electrode ( 34 ). In particular, the outer electrode includes an ion inlet ( 38 ) for introducing ions into a first portion of the analyzer region and an ion outlet ( 56 ) for extracting ions from a second portion of the analyzer region. The FAIMS is characterized in that a gas-directing conduit ( 48 ) is provided through at least a portion of the inner electrode. In particular, the gas-directing conduit includes an opening at a first end thereof for supporting fluid communication between the gas-directing conduit and the first portion of the analyzer region. The gas-directing conduit is adapted at a second end thereof opposite the first end for supporting fluid communication between a gas source and the gas-directing conduit. The opening is orientated relative to the ion inlet such that gas provided through the gas-directing conduit flows partially outwardly from the analyzer region through the ion inlet, so as to desolvate the ions produced at an ionization source ( 42, 45 ) as they are introduced into the analyzer region.

FIELD OF THE INVENTION

The instant invention relates generally to high field asymmetricwaveform ion mobility spectrometry (FAIMS), more particularly theinstant invention relates to a FAIMS electrode assembly for supportingdesolvation of electrosprayed ions absent a separate desolvationchamber.

BACKGROUND OF THE INVENTION

High sensitivity and amenability to miniaturization for field-portableapplications have helped to make ion mobility spectrometry (IMS) animportant technique for the detection of many compounds, includingnarcotics, explosives, and chemical warfare agents as described, forexample, by G. Eiceman and Z. Karpas in their book entitled “IonMobility Spectrometry” (CRC, Boca Raton, 1994). In IMS, gas-phase ionmobilities are determined using a drift tube with a constant electricfield. Ions are separated in the drift tube on the basis of differencesin their drift velocities. At low electric field strength, for example200 V/cm, the drift velocity of an ion is proportional to the appliedelectric field strength and the mobility, K, which is determined fromexperimentation, is independent of the applied electric field.Additionally, in IMS the ions travel through a bath gas that is atsufficiently high pressure that the ions rapidly reach constant velocitywhen driven by the force of an electric field that is constant both intime and location. This is to be clearly distinguished from thosetechniques, most of which are related to mass spectrometry, in which thegas pressure is sufficiently low that, if under the influence of aconstant electric field, the ions continue to accelerate.

E. A. Mason and E. W. McDaniel in their book entitled “TransportProperties of Ions in Gases” (Wiley, New York, 1988) teach that at highelectric field strength, for instance fields stronger than approximately5,000 V/cm, the ion drift velocity is no longer directly proportional tothe applied electric field, and K is better represented by K_(H), anon-constant high field mobility term. The dependence of K_(H) on theapplied electric field has been the basis for the development of highfield asymmetric waveform ion mobility spectrometry (FAIMS). Ions areseparated in a FAIMS analyzer region on the basis of a difference in themobility of an ion at high field strength, K_(H), relative to themobility of the ion at low field strength, K. In other words, the ionsare separated due to the compound dependent behavior of K_(H) as afunction of the applied electric field strength.

In general, a device for separating ions according to the FAIMSprinciple has an analyzer region that is defined by a space betweenfirst and second spaced-apart electrodes. The first electrode ismaintained at a selected do voltage, often at ground potential, whilethe second electrode has an asymmetric waveform V(t) applied to it. Theasymmetric waveform V(t) is composed of a repeating pattern including ahigh voltage component, V_(H), lasting for a short period of time t_(H)and a lower voltage component, V_(L), of opposite polarity, lasting alonger period of time t_(L). The waveform is synthesized such that theintegrated voltage-time product, and thus the field-time product,applied to the second electrode during each complete cycle of thewaveform is zero, for instance V_(H) t_(H)+V_(L) t_(L)=0; for example+2000 V for 10 μs followed by −1000 V for 20 μs. The peak voltage duringthe shorter, high voltage portion of the waveform is called the“dispersion voltage” or DV, which is identically referred to as theapplied asymmetric waveform voltage.

Generally, the ions that are to be separated are entrained in a streamof gas flowing through the FAIMS analyzer region, for example between apair of horizontally oriented, spaced-apart electrodes. Accordingly, thenet motion of an ion within the analyzer region is the sum of ahorizontal x-axis component due to the stream of gas and a transversey-axis component due to the applied electric field. During the highvoltage portion of the waveform an ion moves with a y-axis velocitycomponent given by v_(H)=K_(H)E_(H), where E_(H) is the applied field,and K_(H) is the high field ion mobility under operating electric field,pressure and temperature conditions. The distance traveled by the ionduring the high voltage portion of the waveform is given byd_(H)=v_(H)t_(H)=K_(H)E_(H)t_(H), where t_(H) is the time period of theapplied high voltage. During the longer duration, opposite polarity, lowvoltage portion of the asymmetric waveform, the y-axis velocitycomponent of the ion is v_(L)=KE_(L), where K is the low field ionmobility under operating pressure and temperature conditions. Thedistance traveled is d_(L)=v_(L)t_(L)=KE_(L)t_(L). Since the asymmetricwaveform ensures that (V_(H) t_(H))+(V_(L) t_(L))=0, the field-timeproducts E_(H)t_(H) and E_(L)t_(L) are equal in magnitude. Thus, ifK_(H) and K are identical, d_(H) and d_(L) are equal, and the ion isreturned to its original position along the y-axis during the negativecycle of the waveform. If at E_(H) the mobility K_(H)>K, the ionexperiences a net displacement from its original position along they-axis. For example, if a positive ion travels farther during thepositive portion of the waveform, for instance d_(H)>d_(L), then the ionmigrates away from the second electrode and eventually will beneutralized at the first electrode.

In order to reverse the transverse drift of the positive ion in theabove example, a constant negative dc voltage is applied to the secondelectrode. The difference between the dc voltage that is applied to thefirst electrode and the dc voltage that is applied to the secondelectrode is called the “compensation voltage” (CV). The CV voltageprevents the ion from migrating toward either the second or the firstelectrode. If ions derived from two compounds respond differently to theapplied high strength electric fields, the ratio of K_(H) to K may bedifferent for each compound. Consequently, the magnitude of the CV thatis necessary to prevent the drift of the ion toward either electrode isalso different for each compound. Thus, when a mixture including severalspecies of ions, each with a unique K_(H)/K ratio, is being analyzed byFAIMS, only one species of ion is selectively transmitted to a detectorfor a given combination of CV and DV. In one type of FAIMS experiment,the applied CV is scanned with time, for instance the CV is slowlyramped or optionally the CV is stepped from one voltage to a nextvoltage, and a resulting intensity of transmitted ions is measured. Inthis way a CV spectrum showing the total ion current as a function ofCV, is obtained.

U.S. Pat. No. 5,420,424, issued to Carnahan and Tarassov on May 30,1995, teaches a FAIMS device having cylindrical electrode geometry andelectrometric ion detection, the contents of which are incorporatedherein by reference. The FAIMS analyzer region is defined by an annularspace between inner and outer cylindrical electrodes. In use, ions thatare to be separated are entrained into a flow of a carrier gas and arecarried into the analyzer region via an ion inlet orifice. Once insidethe analyzer region, the ions become distributed all the way around theinner electrode as a result of the carrier gas flow and ion-ionrepulsive forces. The ions are selectively transmitted within theanalyzer region to an ion extraction region at an end of the analyzerregion opposite the ion inlet end. In particular, a plurality of ionoutlet orifices is provided around the circumference of the outerelectrode for extracting the selectively transmitted ions from the ionextraction region for electrometric detection. Of course, theelectrometric detectors provide a signal that is indicative of the totalion current arriving at the detector. Accordingly, the CV spectrum thatis obtained using the Carnahan device does not include informationrelating to an identity of the selectively transmitted ions. It is alimitation of the Carnahan device that the peaks in the CV spectrum arehighly susceptible to being assigned incorrectly.

Replacing the electrometric detector with a mass spectrometer detectionsystem provides an opportunity to obtain additional experimental datarelating to the identity of ions giving rise to the peaks in a CVspectrum. For instance, the mass-to-charge (m/z) ratio of ions that areselectively transmitted through the FAIMS at a particular combination ofCV and DV can be measured. Additionally, replacing the mass spectrometerwith a tandem mass spectrometer makes it possible to perform afull-fledged structural investigation of the selectively transmittedions. Unfortunately, the selectively transmitted ions are difficult toextract from the analyzer region of the Carnahan device for subsequentdetection by a mass spectrometer. In particular, the orifice plate of amass spectrometer typically includes a single small sampling orifice forreceiving ions for introduction into the mass spectrometer. Thisrestriction is due to the fact that a mass spectrometer operates at amuch lower pressure than the FAIMS analyzer. In general, the size of thesampling orifice into the mass spectrometer is limited by the pumpingefficiency of the mass spectrometer vacuum system. In principle, it ispossible to align the sampling orifice of a mass spectrometer with asingle opening in the FAIMS outer electrode of the Carnahan device;however, such a combination suffers from very low ion transmissionefficiency and therefore poor detection limits. In particular, theCarnahan device does not allow the selectively transmitted ions to beconcentrated for extraction through the single opening. Accordingly,only a small fraction of the selectively transmitted ions are extractedfrom the analyzer region, the vast majority of the selectivelytransmitted ions being neutralized eventually upon impact with anelectrode surface.

Guevremont et al. describe the use of curved electrode bodies, forinstance inner and outer cylindrical electrodes, for producing atwo-dimensional atmospheric pressure ion focusing effect that results inhigher ion transmission efficiencies than can be obtained using, forexample, a FAIMS device having parallel plate electrodes. In particular,with the application of an appropriate combination of DV and CV an ionof interest is focused into a band-like region between the cylindricalelectrodes as a result of the electric fields which change with radialdistance. Focusing the ions of interest has the effect of reducing thenumber of ions of interest that are lost as a result of the ionsuffering a collision with one of the inner and outer electrodes.

In WO 00/08455, the contents of which are incorporated herein byreference, Guevremont and Purves describe an improved tandem FAIMS/MSdevice, including a domed-FAIMS analyzer. In particular, the domed-FAIMSanalyzer includes a cylindrical inner electrode having a curved surfaceterminus proximate the ion outlet orifice of the FAIMS analyzer region.The curved surface terminus is substantially continuous with thecylindrical shape of the inner electrode and is aligned co-axially withthe ion outlet orifice. During use, the application of an asymmetricwaveform to the inner electrode results in the normal ion-focusingbehavior as described above, except that the ion-focusing action extendsaround the generally spherically shaped terminus of the inner electrode.This causes the selectively transmitted ions to be directed generallyradially inwardly within the region that is proximate the terminus ofthe inner electrode. Several contradictory forces are acting on the ionsin this region near the terminus of the inner electrode. The force ofthe carrier gas flow tends to influence the ion cloud to travel towardsthe ion-outlet orifice, which advantageously also prevents the ions frommigrating in a reverse direction, back towards the ionization source.Additionally, the ions that get too close to the inner electrode arepushed back away from the inner electrode, and those near the outerelectrode migrate back towards the inner electrode, due to the focusingaction of the applied electric fields. When all forces acting upon theions are balanced, the ions are effectively captured in every direction,either by forces of the flowing gas, or by the focusing effect of theelectric fields of the FAIMS mechanism. This is an example of a threedimensional atmospheric pressure ion trap, as described in greaterdetail by Guevremont and Purves in WO 00/08457, the contents of whichare incorporated herein by reference.

Guevremont and Purves further disclose a near-trapping mode of operationfor the above-mentioned tandem FAIMS/MS device, which achieves iontransmission from the domed-FAIMS to a mass spectrometer with highefficiency. Under near-trapping conditions, the ions that accumulate inthe three-dimensional region of space near the spherical terminus of theinner electrode are caused to leak from this region, being pulled by aflow of gas towards the ion-outlet orifice. The ions that leak out fromthis region do so as a narrow, approximately collimated beam, which ispulled by the gas flow through the ion-outlet orifice and into a smallerorifice leading into the vacuum system of the mass spectrometer.Accordingly, such tandem FAIMS/MS devices are highly sensitiveinstruments that are capable of detecting and identifying ions ofinterest at part-per-billion levels.

It is known that certain types of ionization sources, such as forinstance an electrospray ionization source (ESI), produce ions that arehighly solvated. When these highly solvated ions are introduced into theFAIMS analyzer region, some of the solvent evaporates from around theion, thereby contaminating the carrier gas that is flowing through theanalyzer region. Unfortunately, FAIMS is highly sensitive to moistureand contaminants in the gas entering the analyzer region. In fact, it isusual that contaminants, or too much water vapour, will result incomplete loss of signal and failure of the FAIMS to function properly.Since electrospray ionization involves thehigh-voltage-assisted-atomization of a solvent mixture, the amount ofwater and other volatile solvents is far too high to be tolerated in theFAIMS. For this reason, the prior art ESI-FAIMS combination includes atype of solvent removal process embodied by the curtain gas, orcounter-current gas flow, to prevent neutral solvent molecules fromentering the FAIMS analyzer.

In WO 00/08455, the contents of which are incorporated herein byreference, Guevremont et al. teach an ESI-FAIMS combination including asmall chamber disposed between and separating the FAIMS from theelectrospray ionization source. The small chamber includes a gas inletand a gas outlet, for providing a gas flow through the small chamber ina direction that is approximately transverse to the direction in whichthe ions are directed between the ESI and an inlet orifice of the FAIMS.A portion of the gas flow, which portion is referred to as thecounter-current of gas, enters the ESI chamber so as to desolvate theions and to carry the neutral solvent molecules away from the FAIMS andout of the ESI chamber via an outlet port thereof. Accordingly, theneutral solvent molecules are prevented from entering the vicinity ofthe entrance to the FAIMS. Unfortunately, the separate chamber adds tothe overall complexity of the device, increases space requirements ofthe device, requires separate gas flow connections, and likely reducesthe efficiency of introducing ions into the FAIMS since the gas flow isprovided through the separate chamber in a direction that is transverseto the direction in which the ions are traveling. In a side-to-sideFAIMS device, the need for a separate desolvation chamber may negate oneof the advantages of the side-to-side FAIMS device, that is, a shorteneddistance between the ion inlet and the ion outlet of the analyzerregion, which allows the side-to-side FAIMS device to be used wherespace considerations are particularly important.

Guevremont et al. further teach in WO 00/08455 that the counter-currentof gas can be achieved by adjusting the FAIMS analyzer gas flow, so thatsome of the gas exits the FAIMS analyzer region through the ion inletorifice. In this way, the introduction of neutral contaminants into theanalyzer region is also avoided. Advantageously, adjusting the FAIMSanalyzer gas flow may result in higher ion transmission than is achievedusing a separate chamber, however, Guevremont et al. also caution thatif the analyzer gas flow is adjusted accidentally, such that the gasfrom the ESI chamber is passed into the FAIMS analyzer region, then theperformance of the FAIMS may be severely compromised for a period oftime, possibly a number of hours, after the accident occurs.Furthermore, some FAIMS electrode geometries, such as for instance theside-to-side FAIMS, use a common inlet for the introducing the ions andthe carrier gas into the analyzer region. Accordingly, it is notpossible to adjust the FAIMS analyzer gas flow so that some of the gasexits the FAIMS analyzer region through the ion inlet orifice, asdescribed above.

In addition, adjusting the FAIMS analyzer gas flow so that some of thegas exits the FAIMS analyzer region through the ion inlet orificeimposes an undesirable limitation upon the operational flexibility ofthe FAIMS. For instance, it is difficult to adjust a flow rate of theanalyzer gas so as to optimize conditions for trapping ions in adomed-FAIMS analyzer when the same adjustment also affects the iondesolvation efficiency. Similarly, an optimum analyzer gas flow rate fordesolvating ions may result in unacceptably rapid transmission of theions through the analyzer region, independent of the electrode geometry,and thereby result in incomplete ion separation.

It would be advantageous to provide a compact and inexpensive system fordesolvating ions for introduction into a FAIMS analyzer that overcomesthe limitations of the prior art. It would be further advantageous toprovide a system for desolvating ions that is adaptable for use withFAIMS devices having a plurality of different electrode geometries.Preferably, the system for desolvating ions is reliable and userfriendly, so as to avoid accidental contamination of the FAIMS analyzerby neutral solvent molecules or contaminants.

SUMMARY OF THE INVENTION

In accordance with an aspect of the instant invention there is providedan apparatus for separating ions, comprising: a high field asymmetricwaveform ion mobility spectrometer comprising an analyzer region definedby a space between an inner electrode and an outer electrode, the outerelectrode defining an ion inlet for introducing ions into a firstportion of the analyzer region and an ion outlet for extracting ionsfrom a second portion of the analyzer region; Characterized in that: agas-directing conduit is provided through at least a portion of one ofthe inner electrode and the outer electrode, the gas-directing conduithaving an opening at a first end thereof for supporting fluidcommunication between the gas-directing conduit and the first portion ofthe analyzer region, the gas-directing conduit being adapted at a secondend thereof opposite the first end for supporting fluid communicationbetween a gas source and the gasdirecting conduit.

In accordance with an aspect of the instant invention there is providedan apparatus for separating ions, comprising: an inner electrode havinga length and an outer surface that is curved in a direction transverseto the length, the inner electrode comprising a gas outlet within thecurved outer surface and at least a gas inlet, the gas outlet being influid communication with the at least a gas inlet via an interiorportion of the inner electrode, for introducing a flow of a gas providedthrough the at least a gas inlet into the analyzer region; an outerelectrode having a length, a channel extending therethrough along atleast a portion of the length, and a curved inner surface, the outerelectrode being approximately coaxially aligned with the innerelectrode, a portion of the length of the outer electrode overlapping aportion of the length of the inner electrode and forming an analyzerregion therebetween, the outer electrode comprising an ion inlet withina first portion of the curved inner surface for introducing ions from anionization source into the analyzer region and an ion outlet within asecond portion of the curved inner surface for extracting ions from theanalyzer region; and, an electrical controller for applying anasymmetric waveform voltage to at least one of the inner electrode andouter electrode and for applying a direct current compensation voltageto at least one of the inner electrode and outer electrode.

In accordance with another aspect of the instant invention there isprovided a method for separating ions, comprising the steps of:providing a FAIMS analyzer region defined by a space between inner andouter spaced apart electrodes; producing ions at an ionization sourcethat is in fluid communication with the analyzer region via an ion inletwithin the outer electrode; introducing the ions produced at anionization source into the FAIMS analyzer region via the ion inletwithin the outer electrode; providing a flow of a gas into the analyzerregion through at least a first portion of the inner electrode such thata first portion of the flow of a gas flows out of the analyzer regionthrough the ion inlet.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described inconjunction with the following drawings, in which similar referencenumbers designate similar items:

FIG. 1 is an end view of a prior art side-to-side FAIMS including aseparate chamber for desolvating electrosprayed ions;

FIG. 2 a is an end view of a side-to-side FAIMS including an innerelectrode having a gas directing conduit according to the instantinvention;

FIG. 2 b is a simplified side cross-sectional view of the innerelectrode of FIG. 2 a;

FIG. 2 c is a simplified side cross-sectional view of another innerelectrode that is suitable for use with the side-to-side FAIMS of FIG. 2a;

FIG. 2 d is a simplified side cross-sectional view of still anotherinner electrode that is suitable for use with the side-to-side FAIMS ofFIG. 2 a;

FIG. 3 is an end view of another side-to-side FAIMS including an innerelectrode having a gas directing conduit according to the instantinvention;

FIG. 4 a is an end view of still another side-to-side FAIMS including aninner electrode having a gas directing conduit according to the instantinvention, showing gas flows during a first mode of operation;

FIG. 4 b is a simplified side cross-sectional view of the innerelectrode of FIG. 4 a, showing gas flows during the first mode ofoperation;

FIG. 4 c is an end view of the side-to-side FAIMS shown at FIG. 4 a,showing gas flows during a second mode of operation;

FIG. 4 d is a simplified side cross-sectional view of the innerelectrode of FIG. 4 b, showing gas flows during the second mode ofoperation;

FIG. 5 a is an end view of yet another side-to-side FAIMS including atubular inner electrode containing a separate gas directing tubeaccording to the instant invention;

FIG. 5 b is a simplified side cross-sectional side view of the innerelectrode of FIG. 5 a;

FIG. 6 a is an end view of yet another side-to-side FAIMS including atubular inner electrode, absent a separate gas directing tube, accordingto the instant invention;

FIG. 6 b is a simplified side cross-sectional side view of the innerelectrode of FIG. 6 a;

FIG. 7 is a simplified block diagram of a domed-FAIMS analyzer includingan inner electrode having a gas-directing conduit according to theinstant invention; and,

FIG. 8 is a simplified flow diagram for a method of separating ionsaccording to the instant invention;

FIG. 9 a is a partial cross sectional view of a domed-FAIMS apparatusincluding an outer electrode having a gas-directing conduit according tothe instant invention;

FIG. 9 b is a partial cross sectional view of another domed-FAIMSapparatus including an outer electrode having a gas-directing conduitaccording to the instant invention;

FIG. 10 a is an end view of a side-to-side FAIMS including an outerelectrode having a gas directing conduit according to the instantinvention;

FIG. 10 b is a simplified side elevational view of another side-to-sideFAIMS including an outer electrode having a gas directing conduitaccording to the instant invention; and,

FIG. 11 an end view of another side-to-side FAIMS including an outerelectrode having a gas-directing conduit according to the instantinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description is presented to enable a person skilled in theart to make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andthe scope of the invention. Thus, the present invention is not intendedto be limited to the embodiments disclosed, but is to be accorded thewidest scope consistent with the principles and features disclosedherein.

Referring to FIG. 1, shown is an end view of a prior art side-to-sideFAIMS device including a separate chamber for desolvating electrosprayedions. The side-to-side FAIMS device, shown generally at 10, includesinner and outer cylindrical electrodes 12 and 14, respectively, whichare supported by an electrically insulating material 15 in anoverlapping, spaced-apart arrangement. The generally annular spacebetween the inner electrode 12 and the outer electrode 14 defines aFAIMS analyzer region 16. The analyzer region 16 is of approximatelyuniform width and extends around the circumference of the innerelectrode 12. The inner electrode 12 is in electrical communication witha power supply (not shown) that during use is capable of applying a highvoltage asymmetric waveform (DV) and a low voltage do compensationvoltage (CV) to the inner electrode 12.

An ion inlet 18 is provided through the outer electrode 14 forintroducing ions from an ion source into the analyzer region 16. The ionsource is in the form of an electrospray ionization ion source includinga liquid delivery capillary 24 and a fine-tipped electrospray needle 22that is held at high voltage. The electrospray needle 22 is containedwithin an electrospray ionization (ES) chamber 23 having a gas outlet27. The ionization source further includes a curtain plate 26 serving asa counter-electrode for the electrospray needle 22. An orifice 25 withinthe curtain plate electrode 26 allows for transmission of ions producedat the electrospray needle 22 into a separate chamber 29. A flow of agas, which is represented in FIG. 1 by a series of closed-headed arrows,is provided through a gas inlet 28 into the separate chamber 29. A firstportion of the gas flows into the analyzer region 16, to carry the ionsaround the inner electrode 12 and toward an ion outlet 20. The orifice25 within the curtain plate 26 allows for the flow of a second portionof the gas in a direction that is counter-current to the direction inwhich the ions are traveling in the separate chamber 29 towards the ioninlet 18, so as to desolvate the ions before they are introduced intothe analyzer region 16. The flow of the second portion of the gas exitsthe ESI chamber via gas outlet 27, thereby removing the solvent vapourand preventing the contamination of the FAIMS analyzer region 16.

Once inside the FAIMS analyzer region 16, the ions are carried throughan electric field that is formed within the FAIMS analyzer region 16 bythe application of the DV and the CV to the inner electrode 12. Ionseparation occurs within the FAIMS analyzer region 16 on the basis ofthe high field mobility properties of the ions. Those ions that have astable trajectory for a particular combination of DV and CV areselectively transmitted through the FAIMS analyzer region 16, whilstother ions of the mixture collide with an electrode surface and arelost. The selectively transmitted ions are extracted from the analyzerregion 16 via ion outlet orifice 20 and are typically subjected to oneof detection and further analysis.

As will be obvious to one of skill in the art, the ion inlet orifice 18additionally functions as a carrier gas inlet into the analyzer region16. Accordingly, the ions and the carrier gas are co-introduced into aprior art side-to-side FAIMS, and travel in a same direction through theanalyzer region 16 toward the ion outlet orifice 20. Since no separatecarrier gas inlet is provided into a prior art side-to-side FAIMS, it isnot possible to direct a flow of gas counter-current to the directionthe ions are traveling in the vicinity of the ion inlet orifice 18. As aconsequence, providing the separate chamber 29 to support desolvation ofthe ions before they are introduced into the analyzer region of aside-to-side FAIMS is critically important to the operaion of theelectrospray ionization source when used in conjunction with a FAIMSanalyzer.

Referring now to FIG. 2 a, shown is an end view of a side-to-side FAIMSincluding an inner electrode having a gas-directing conduit, such as forinstance a bored-out channel, according to the instant invention. Theside-to-side FAIMS device, shown generally at 30, includes inner andouter cylindrical electrodes 32 and 34, respectively, which aresupported by an electrically insulating material 35 in an overlapping,spaced-apart arrangement The generally annular space between the innerelectrode 32 and the outer electrode 34 defines a FAIMS analyzer region36. The analyzer region 36 is of approximately uniform width and extendsaround the circumference of the inner electrode 32. The inner electrode32 is in electrical communication with a power supply (not shown) thatduring use is capable of applying a high voltage asymmetric waveformvoltage (DV) and a low voltage dc compensation voltage (CV) to the innerFAIMS electrode 32.

An ion inlet 38 is provided through the outer electrode 34 forintroducing ions from an ion source into the analyzer region 36. The ionsource is in the form of an electrospray ionization (ESI) sourceincluding a liquid delivery capillary 44 and a fine-tipped electrosprayneedle 42 that is held at high voltage. An optional ionization cylinder45 is sealed gas tight against the insulating material 35 to define anESI chamber 43 that contains the electrospray needle 42. The ionizationcylinder 45 includes at least a gas outlet 47 for allowing gas to exitfrom the ESI chamber 43. The ionization cylinder 45, when present, isoptionally fabricated from one of an insulating material and aconductive material. If the ionization cylinder 45 is conductive, it isoptionally in electrical communication with the outer electrode 34. Ofcourse, the ionization cylinder 45 is sufficiently large such that itdoes not have an adverse effect on the electrospray ionization process,for example as a result of an electrical discharge between the tip ofthe electrospray needle 42 and the ionization cylinder 45.

Referring still to FIG. 2 a, the ESI chamber is provided in direct fluidcommunication with the analyzer region 36. In particular, a separatechamber similar to drawing element 29 of FIG. 1 is absent in the FAIMSdevice that is shown generally at 30 in FIG. 2 a. Accordingly, the outerelectrode 34 serves as the counter-electrode of the electrospray source.Some of the ions that are produced by the electrospray needle 42 passthrough the ion inlet 38 and into the analyzer region 36. However, theneutral solvent vapour cannot enter the analyzer region 36. This is madepossible by the dual functionality of the inner electrode 32. As wasdescribed with reference to FIG. 1, the asymmetric waveform ispreferentially applied to the inner electrode 32. In addition, a conduit48 is provided through the core of the inner electrode 32 to supportpassage of a flow of a gas through the inner electrode 32 and into theanalyzer region 36. To this end, an outlet 52 is provided between theconduit 48 and the analyzer region 36 for directing the flow of gas fromthe conduit 48 through an opening 54 within the surface of the innerelectrode 32, and finally into the analyzer region 36. For instance, theoutlet 52 is a hole that is drilled approximately radially into theinner electrode 32 and that is in fluid communication with the conduit48, as is shown in FIG. 2 b. Preferably, the opening 54 of the outlet 52in the inner electrode 32 is sufficiently large to prevent a highvelocity, high turbulence gas flow, which could adversely affect theefficiency of ion transport into the FAIMS analyzer region 36.

During use, gas is introduced into the port 50 from a not illustratedgas source. The gas flows longitudinally within the conduit 48 throughthe inner electrode 32, is diverted through the outlet 52, passes out ofthe inner electrode 32 via the opening 54, and enters into the FAIMSanalyzer region 36 in the vicinity of the ion inlet 38. As is shownschematically by the closed-headed arrows in FIG. 2 a, if the volume ofgas introduced through the conduit 48 in the inner electrode 32 exceedsthe volume of gas flowing out of the analyzer region 36 through an ionoutlet 56, then the gas will split into two flows comprising adesolvation gas and a carrier gas. The desolvation gas flows through theion inlet orifice 38 in a direction counter-current to the direction theions are traveling, continuing toward the electrospray needle 42 in theESI chamber 43, and eventually exiting the ESI chamber 43 via one of thegas outlets 47. The carrier gas flows approximately equally around eachside of the inner electrode 32, and out through the ion outlet 56, as isillustrated by the arrows in FIG. 2 a.

Advantageously, the high voltage that is applied to the ionizationsource results in a strong electric field that directs electrosprayedions away from the electrospray needle 42 and toward thecounter-electrode, in this instance the outer electrode. Some of theions enter the analyzer region 36 through the ion inlet 38, through thecountercurrent desolvation gas flow, and into the carrier gas flowstream that transports ions through the analyzer region 36 to the ionoutlet 56. The dc voltage difference between the inner electrode 32 andthe outer electrode 34, in other words the CV, acts to pull the ionsthrough the ion inlet 38 into the analyzer region 36. Of course, theelectric fields on either side of the ion inlet 38 penetrate through theinlet and add to the field on the other side of the ion inlet. Thedesolvation gas that flows toward the electrospray needle 42 through theion inlet 38 aids in desolvation and prevents solvent and other neutralsfrom entering the ion inlet 38. Further advantageously, theelectrosprayed ions are desolvated without the need for a separatedesolvation chamber, thereby allowing a more efficient transfer of ionsinto the FAIMS device 30.

Preferably, the ion inlet orifice 38 is sufficiently large that thefields from the ionization region are able to penetrate through the ioninlet orifice 38, so as to assist the travel of the ions through thecountercurrent gas flow, or desolvation gas, and into the analyzerregion where the ions become entrained in the carrier gas flow thattransports the ions through the analyzer region 36 to the ion outlet 56.Optionally, at least one of the ion inlet 38 and the ion outlet 56 isprovided as a slit-shaped opening in the outer electrode 34.

In the embodiment that is described with reference to FIGS. 2 a and 2 b,the conduit 48 is aligned generally along the longitudinal axis of theinner electrode 32, intersecting the outlet 52 at right angles. Ofcourse, other configurations of the conduit/outlet may also beenvisaged. Referring now to FIG. 2 c, shown is a first optionalconfiguration in which the conduit 48 and an outlet 58 intersect atother than right angles. In particular, the outlet 58 is angled upwardlyin the figure, so as to direct in a predetermined direction a flow ofgas through an opening 60 of the outlet 58. Referring now to FIG. 2 d,shown is a second optional configuration in which the conduit 48intersects a funnel shaped outlet 62, which enlarges in a direction thatis generally radially away from the longitudinal axis of the innerelectrode. Accordingly, a gas flow through the conduit 48 enlarges as itmoves through the outlet 62, such that the gas flow rate decreases andthe gas flow spreads out through the opening 64 into a larger portion ofthe analyzer region near the ion inlet 38. Of course, the opening 64affects the electric fields in an adjacent portion of the analyzerregion. Accordingly, the size of the opening 64 is selected so as not tonegatively affect the performance of the FAIMS analyzer.

Optionally, the conduit 48 is disposed other than generally along thelongitudinal axis of the inner electrode 32, or even at an angle to thelongitudinal axis. Further optionally, the conduit 48 includes a notillustrated plurality of gas inlets, each gas inlet of the plurality ofgas inlets in communication with a source of a different gas.

Referring now to FIG. 3, shown is an end view of another side-to-sideFAIMS including an inner electrode having a gas-directing conduit, suchas for instance a bored-out channel, according to the instant invention.The side-to-side FAIMS device, shown generally at 70, includes inner andouter generally cylindrical electrodes 72 and 74, respectively, whichare supported by an electrically insulating material 76 in anoverlapping, spaced-apart arrangement. The inner electrode 72 is inelectrical communication with a power supply (not shown) that during useis capable of applying a high voltage asymmetric waveform (DV) and a lowvoltage dc compensation voltage (CV) to the inner electrode 72.

The generally annular space between the inner electrode 72 and the outerelectrode 74 defines a FAIMS analyzer region 78. In the device 70, theinner electrode 72 is modified so that a protruding part 80 of the innerelectrode 72 forms a gas tight seal with the electrically insulatingmaterial 76, thereby forcing the gas flow, which is represented in thefigure by a series of closed headed arrows, around one side of the innerelectrode 72 toward an ion outlet 82. Optionally, the protruding part 80of the inner electrode 72 is replaced by a protruding section of theelectrically insulating material 76, which extends radially inwardlytoward the inner electrode 72. When a protruding section of theelectrically insulating material 76 is used to control the direction ofgas flows around the circumference of the inner electrode 72, the sizeof an opening 84 in the outer electrode 74 is optionally reduced, sincethe electrically insulating material 76 does not readily support anelectrical discharge. Preferably, however, the design of a protrudingsection that is fabricated from the electrically insulating material 76must avoid, or at least minimize, the occurrences of electricaldischarge and the subsequent creation of burn tracks in the electricallyinsulating material.

An ion inlet 86 is provided through the outer electrode 74 forintroducing ions from an ion source into the analyzer region 78. The ionsource is in the form of an electrospray ionization (ESI) sourceincluding a liquid delivery capillary 88 and a fine-tipped electrosprayneedle 90 that is held at high voltage. An optional ionization cylinder92 is sealed gas tight against the insulating material 76 to define anESI chamber 94 that contains the electrospray needle 90. The ionizationcylinder 92 includes at least a gas outlet 96 for allowing gas to exitfrom the ESI chamber 94. The ionization cylinder 92, when present, isoptionally fabricated from one of an insulating material and aconductive material. If the ionization cylinder 92 is conductive, it isoptionally in electrical communication with the outer electrode 74. Ofcourse, the ionization cylinder 92 is sufficiently large such that itdoes not have an adverse effect on the electrospray ionization process,for example as a result of an electrical discharge between the tip ofthe electrospray needle 90 and the ionization cylinder 92.

Referring still to FIG. 3, the ESI chamber 94 is provided in directfluid communication with the analyzer region 78. In particular, aseparate chamber similar to drawing element 29 of FIG. 1 is absent inthe FAIMS device that is shown generally at 70 in FIG. 3. Accordingly,the outer electrode 74 serves as the counter-electrode of theelectrospray source. Some of the ions that are produced by theelectrospray needle 90 pass through the ion inlet 86 and into theanalyzer region 78. However, the neutral solvent vapour cannot enter theanalyzer region 78. This is made possible by the dual functionality ofthe inner electrode 72. As was described with reference to FIG. 1, theasymmetric waveform is preferably applied to the inner electrode 72. Inaddition, a conduit 98 is provided through the core of the innerelectrode 72 to support passage of a flow of a gas through the innerelectrode 72 and into the analyzer region 78. To this end, an outlet 100is provided between the conduit 98 and the analyzer region 78 fordirecting the flow of gas from the conduit 98 through an opening 102within the surface of the inner electrode 72, and finally into theanalyzer region 78. For instance, the outlet 100 is a hole that isdrilled approximately radially into the inner electrode 72 and that isin fluid communication with the conduit 98. Preferably, the opening 102of the outlet 100 in the inner electrode 72 is sufficiently large toprevent a high velocity, high turbulence gas flow, which could adverselyaffect the efficiency of ion transport into the FAIMS analyzer region78.

During use, gas from a not illustrated gas source is introduced via aport 104 into the conduit 98. The gas flows longitudinally within theconduit 98 through the inner electrode 72, is diverted through theoutlet 100, passes out of the inner electrode 72 via the opening 102,and enters into the FAIMS analyzer region 78. As is shown schematicallyby the closed-headed arrows in FIG. 3, if the volume of gas that isintroduced through the conduit 98 in the inner electrode 72 exceeds thevolume of gas flowing out of the analyzer region 78 through the ionoutlet 82, then the gas will split into two flows comprising adesolvation gas and a carrier gas. The desolvation gas flows through theion inlet orifice 86 in a direction counter-current to the direction theions are traveling, continuing toward the electrospray needle 90 in theESI chamber 94, and eventually exiting the ESI chamber 94 via one of thegas outlets 96. The carrier gas flows in one direction only around thecircumference of the inner electrode 72, being blocked in the otherdirection by the protruding part 80. The carrier gas flowing in the onedirection exits the analyzer region via the ion outlet orifice 82.

The location around the circumference of the inner electrode 72 at whichthe gas emerges from the opening 102 and enters the analyzer region 78is a critical consideration. In the device 70, the opening 102 islocated, in a circumferential direction, between the ion inlet 86 andthe protruding part 80. Such an arrangement provides a gas flow exitingthe analyzer region 78 through the ion inlet 86 that is suitable fordesolvating the ions produced at the electrospray needle 90 and anothergas flow for carrying the desolvated ions through the analyzer regiontoward the ion outlet orifice 82. Optionally, the opening 102 ispositioned approximately facing the ion inlet 86, such that gas flowsare created similar to the ones that were described with reference toFIG. 2 a. Of course, the carrier gas flow in the device 70 travels inone direction only around the inner electrode 72, being blocked in theother direction by the protruding part 80. Conversely, a device that issimilar to the one shown generally at 70, but in which the opening 102in the inner electrode 72 is positioned, in a circumferential direction,between the ion inlet 86 and the ion outlet 82, is expected to fail.This device is expected to fail because the electric fields that areproduced by the ionization source do not extend far enough into theanalyzer region 78 to carry an ion through the countercurrent gas flow,such as for example around the inner electrode 72 to a point that isbeyond the opening 102, and into a portion of the gas stream thattransports ions in the direction of the ion outlet 82.

Optionally, when a protruding section of the electrically insulatingmaterial 76 is used to control the direction of gas flows around thecircumference of the inner electrode 72, the inner electrode 72optionally is rotatable about it longitudinal axis. For example, theinner electrode 72 may be rotated through a limited range of positionsrelative to the outer electrode 74, for optimizing the location of theopening 102 relative to the ion inlet 86.

Referring now to FIG. 4 a, shown is an end view of still anotherside-to-side FAIMS including an inner electrode having a gas-directingconduit according to the instant invention, and showing gas flowsproduced during a first mode of operation. Elements labeled with thesame numerals have the same function as those illustrated in FIG. 2 a.The ESI chamber of this embodiment is modified relative to that shown inFIG. 2 a. The ionization cylinder 112 within the ESI chamber 114 doesnot make a gas tight seal against the FAIMS device. Instead, theconductive ionization cylinder 112 is enclosed within a housing 116 thatis secured gas tight against the electrically insulating material 35 ofthe FAIMS device 110. At least a gas inlet 118, two gas inlets 118 beingshown in FIG. 4 a, are provided through the housing 116 at locations forintroducing a gas into the space between the ionization cylinder 112 andthe housing 116. As is illustrated in FIG. 4 a by the series ofclosed-headed arrows, the introduced gas splits into three approximatelyseparate flows on exiting the space between the ionization cylinder 112and the housing 116. Referring now to FIG. 4 b, a first flow, forinstance the sample gas, travels through conduit 48 within the innerelectrode 32 and exits through a port 50 in one of the flat end-surfacesof the inner electrode 32. Referring again to FIG. 4 a, a second gasflow, for instance the desolvation gas, travels toward an ESI needle120, and out through at least one gas outlet 122 from the ionizationchamber 114. A third gas flow, for instance the carrier gas, travelsaround both sides of the inner electrode 32 and toward the ion outlet56. As was described with reference to FIG. 2 a, the electric fieldsfrom the ionization source assist in transporting ions through thecountercurrent gas flow, and into the carrier gas flow that transportsthe ions to the ion outlet 56.

Optionally, the gas flow is controlled by adding a valve and a smallpump to the port 50 in the inner electrode and/or to the gas outlets 122in the ionization chamber 114. Further optionally, the gas flow in theconfiguration that is shown in FIG. 4 a is changed so that the samplegas flow is eliminated and instead gas is introduced to the FAIMS devicethrough both the conduit 48 in the inner electrode 32 and through thegas inlets 118 in the housing 116.

Referring now to FIG. 4 c, shown is an end view of the side-to-sideFAIMS shown at FIG. 4 a, showing gas flows produced during a second modeof operation. Elements labeled with the same numerals have the samefunction as those illustrated in FIGS. 2 a and 4 a When operating in thesecond mode, gas entering through the gas inlets 118 in the housing 116splits into first and second approximately separate flows, as isillustrated by the series of closed-headed arrows in the figure. Thefirst flow travels in a direction toward the electrospray needle 120 andexits through the gas outlets 122 in the ionization chamber 114, therebymaking up a portion of a total desolvation gas flow. The second flowtravels through the ion inlet 124 and around both sides of the innerelectrode 32 toward the ion outlet 56, thereby making up a portion ofthe total carrier gas flow. Now referring also to FIG. 4 d, gas enteringthrough the conduit 48 in the inner electrode 32 splits into third andfourth approximately separate flows, as shown by the series ofclosed-headed arrows in FIGS. 4 c and 4 d. The third flow travelsthrough the ion inlet 124 in a direction toward the electrospray needle120 and exits through the gas outlets 122 in the ionization chamber 114,thereby making up a remaining portion of a total desolvation gas flow.Similarly, the fourth flow travels through the analyzer region aroundboth sides of the inner electrode 32 toward the ion outlet 56, therebymaking up a remaining portion of the total carrier gas flow. As wasdescribed with reference to FIG. 2 a and FIG. 4 a, the electric fieldsfrom the ionization source assist in transporting ions through thecountercurrent gas flow, and into the carrier gas flow that transportsthe ions to the ion outlet 56.

Referring now to FIG. 5 a, shown is an end view of yet anotherside-to-side FAIMS according to the instant invention. Elements labeledwith the same numerals have the same function as those illustrated inFIG. 2 a. Unlike the device 30 that was described with reference toFIGS. 2 a and 2 b, the device shown generally at 150 includes a tubularinner electrode 152 containing a gas-directing conduit in the form of aseparate tube 154. Now referring also to FIG. 5 b, the tube 154 isdisposed inside the tubular inner electrode 152. Preferably, the tube154 forms a gas tight seal with an inner surface of the tubular innerelectrode 152 about an opening 160 through the tubular inner electrode152. The tube 1-54 is also preferably supported by a gas delivery systemat a port 156 of the tube 154. Gas entering the tube 154 through theport 156 emerges into the analyzer region 36 via the opening 160. Thegas flow splits into a first portion flowing through the analyzer region36 and a second portion flowing through the ionization chamber 45, asindicated in the figure by the series of closed-headed arrows, are asdescribed supra with reference to FIG. 2 a. Optionally, the tube 154 isflexibly mounted within the tubular inner electrode 152, such that afree length of the tube 154 is adjustable. Preferably, adjusting thefree length of the tube 154 varies the angle of gas introduction intothe analyzer region.

Referring now to FIG. 6 a, shown is an end view of yet anotherside-to-side FAIMS according to the instant invention. Elements labeledwith the same numerals have the same function as those illustrated inFIG. 5 a Unlike the device 150 that was described with reference toFIGS. 5 a and 5 b, the device shown generally at 170 includes a tubularinner electrode 152 having endfaces, which functions as a gas-directingconduit, absent a separate tube or a bored-through channel. Referringnow also to FIG. 6 b, a port 172 is provided at one of the endfaces ofthe tubular inner electrode 152. The tubular inner electrode 152 issealed gas fight except for the opening 160; Gas entering the interiorvolume of the tubular inner electrode 152 through the port 172 emergesinto the analyzer region 36 via the opening 160. The gas flow splitsinto a first portion flowing through the analyzer region 36 and a secondportion flowing through the ionization chamber 45, as indicated in thefigure by the series of closed-headed arrows, are as described suprawith reference to FIG. 2 a.

The detailed description of the instant invention is provided in termsof a specific and non-limiting example of a particular FAIMS electrodegeometry, specifically a cylindrical side-to-side FAIMS electrodegeometry. Of course, the inventors also envisage alternative embodimentsof the instant invention, which encompass other FAIMS electrodegeometries. Referring now to FIG. 7, shown is a simplified block diagramof one such alternative embodiment, comprising a domed-FAIMS analyzer130 including an inner electrode 132 having a gas-directing conduit 134according to the instant invention. Preferably, the gas-directingconduit 134 intersects an outlet 136, such that an opening 138 of theoutlet 136 is disposed opposite an ion inlet 140 within an outerelectrode 142. If the volume of gas introduced through the conduit 134in the inner electrode 132 exceeds the volume of gas flowing out of ananalyzer region 146 through an ion outlet 148, then the volume ofintroduced gas will split into two approximately separate gas flows,comprising a desolvation gas and a carrier gas, as shown by the seriesof closed-headed arrows in the figure. The desolvation gas flowsoutwardly from the analyzer region 146 through the ion inlet 140 in adirection that is counter-current to the electrosprayed ions, therebyassisting the desolvation of the ions. The carrier gas flows along theanalyzer region in the generally annular space between the inner andouter electrodes, 132 and 142, respectively, and out through the ionoutlet 148. Advantageously, the electrosprayed ions are desolvatedabsent an external desolvation chamber, or curtain plate assembly.

Referring now to FIG. 8, shown is a simplified flow diagram of a methodfor separating ions according to the instant invention. At step 200, aFAIMS analyzer region is provided, the FAIMS analyzer region beingdefined by a space between inner and outer spaced apart electrodes. Atstep 202, ions are produced at an ionization source for introductioninto the analyzer region. In particular, the ionization source is anelectrospray ionization source for producing ions from a sample in aliquid state, such that the produced ions are solvated. At step 204, theions are introduced into the FAIMS analyzer region. In particular, theelectric field from the ionization source extends through an ion inletand into the FAIMS analyzer region, which directs the ions away from theionization source and toward the counter electrode, in this case theouter electrode 142, and thereby assists in transporting ions into theanalyzer region. At step 206, a flow of a gas is provided through aninterior portion of the inner electrode and into the analyzer region,such that a first portion of the flow of a gas flows out of the analyzerregion through the ion inlet and counter-current to a direction thations are traveling in the vicinity of the ion inlet. Advantageously, thecounter-current of gas, or desolvation gas, acts to desolvate the ionsthat are being directed toward the ion inlet by the electric fields fromthe ionization source. Accordingly, the ions are approximatelydesolvated by the time the ions enter the FAIMS analyzer region.Preferably, the ions are desolvated to at least an extent that supportsproper operation of the FAIMS.

Of course, a person of skill in the art realises that the ideasillustrated above are to be generalized to include various shapes ofinner and outer electrodes, as well as various types of electrodesegmentation patterns. Although the invention is described in terms of alimited number of specific embodiments in which the gas-directingconduit is provided through at least a portion of the inner electrode,it will be apparent to the person of skill in the art that some of theabove-mentioned advantages are also achieved by providing agas-directing conduit within a wall portion of an outer FAIMS electrode.For example, as shown in FIG. 9 a, when the wall material of the outerelectrode 300 is sufficiently thick, a channel 302 may be bored througha portion of the outer electrode 300 in a direction that issubstantially perpendicular, over at least a portion of a lengththereof, to a side-wall surface 304 of an ion inlet 306 into theanalyzer region 308. The channel 302 is for providing a flow of a gasthrough the side-wall surface 304 of an ion inlet 306. A portion of theflow of a gas flows in a direction toward an ionization source 312 fordesolvating ions produced at the ionization source 312, and theremaining portion of the flow of a gas flows into the analyzer region308, between the outer electrode 300 and an inner electrode 310.Furthermore, as is shown in FIG. 9 b, the opening of the channel 302within the side-wall portion 304 of the ion inlet 306 is optionallyadapted to selectively direct a portion of the flow of a gas through afirst angled outlet 314 a in a direction toward the ionization source312 for desolvating ions produced thereby, whilst selectively directingthe remaining portion of the flow of a gas through a second angledoutlet 314 b into the analyzer region 308 for transmitting the ionstherethrough.

Referring now to FIG. 10 a, shown is a cross sectional view of anotherFAIMS device according to the instant invention. The FAIMS 400 includesan inner electrode 402, an outer electrode 404, an ion inlet 406 and anion outlet 408. In general, the inner electrode 402 has a length and anouter circumference, whereas the outer electrode 404 has a length and aninner circumference. The inlet 406 and outlet 408 are, for example,provided in the form of one of an orifice and a slit. The components ofthe FAIMS device are embedded in an electrically insulating material410, such as polyetheretherketone (PEEK), which is used for maintainingthe relative position of the electrodes one to the other. Typically, theFAIMS device 400 is in fluid communication with another device, forinstance a not illustrated mass spectrometer detector, so that a gasflow is pulled through the FAIMS device 400 and out of the outlet 408.

Referring still to FIG. 10 a, the FAIMS device 400 comprises a secondinlet, for example a port for a gas inlet 412 through the wall of outerelectrode 404 in the vicinity of the ion inlet 406. Arrows are used inthe figure to illustrate the gas flows through the various portions ofthe FAIMS 400, with longer arrows indicating faster flow rates andshorter arrows indicating relatively slower flow rates. In particular,solid arrows designate a flow of a carrier gas, dotted arrows designatea desolvation gas flow, and dashed arrows designate “extra” gas flow. Afine-tipped electrospray needle 414 that is held at high voltage (powersupply not shown) is only one component of the ionization ion sourceshown at FIG. 10 a. Of course, any other suitable ionization ion sourceis used optionally in place of the electrospray ionization ion source.The gas introduced via the gas inlet 412 into the FAIMS device splitsinto two flows. One of the flows, the extra gas flow 416 travels aroundone side of the inner electrode toward the ion outlet 408. The other gasflow, comprising both the desolvation gas flow 418 and the carrier gasflow 420, travels in a direction around the other side of the innerelectrode toward the ion inlet 406. In a region near the ion inlet 406,the other gas flow further splits into two flows, the desolvation gasflow 418 and the carrier gas flow 420. With sufficient volume of gasflowing outwardly from the FAIMS analyzer region 422 via the ion inlet406, the geometry of the FAIMS device 400 shown at FIG. 10 a provides aflow of gas, the desolvation gas flow 418, in a direction that iscounter-current to the electrosprayed ions that are travelling towardthe ion inlet 406. The desolvation gas flow 418 functions to desolvatethe electrosprayed ions as they travel through the ion inlet 406 towardthe analyzer region 422. This desolvation process reduces the amount ofsolvent and other contaminants that enter the FAIMS analyzer region, andeliminates the need of a curtain plate assembly.

As was described with reference to FIG. 2 a, the high voltage that isapplied to the electrospray needle 414 results in a strong electricfield that directs electrosprayed ions away from the electrospray needle414 and toward the ion inlet 406 in the outer electrode 404, through thecountercurrent desolvation gas flow, and into the analyzer region 422where the ions become entrained in the carrier gas flow stream thattransports the ions through the analyzer region 422 to the ion outlet408. The desolvation gas that flows toward the electrospray needle 414through the ion inlet 406 aids in desolvation thereby preventing solventand other neutrals from entering the ion inlet 406. Advantageously, theelectrosprayed ions are desolvated without the need for a separatedesolvation region, thereby allowing a more efficient transfer of ionsinto the FAIMS device 400.

Referring now to FIG. 10 b, shown is a FAIMS device 500, in which a gasinlet 502 and an ion inlet 504 are positioned at 180° from an ion outlet(not shown). The ion inlet 504 and the gas inlet 502 are adjacent toeach other, but rather than being adjacent along a circumference of acylindrical outer electrode 506, the ion inlet 504 and the gas inlet 502are adjacent to and longitudinally spaced-apart from one another along alength of the outer electrode 506. This positioning of the inletssupports a carrier gas flow 510 around both sides of an inner electrode508, with an approximately same carrier gas flow rate in both directionsaround the inner electrode in a direction toward the outlet. Withsufficient volume of gas flowing outwardly from the FAIMS analyzerregion 512 via the ion inlet 504, the geometry of the FAIMS device 500shown at FIG. 10 b provides a flow of gas, the desolvation gas flow 514,in a direction that is counter-current to the electrosprayed ions thatare travelling toward the ion inlet 504. The desolvation gas flow 514functions to desolvate the electrosprayed ions as they travel throughthe ion inlet 504 toward the analyzer region 512. This desolvationprocess reduces the amount of solvent and other contaminants that enterthe FAIMS analyzer region, and eliminates the need of a curtain plateassembly. Optionally, the gas inlet 502 and the ion inlet 504 are ofdifferent size and or shape.

Referring now to FIG. 11, shown is another FAIMS device according to theinstant invention. Elements labeled with the same numerals have the samefunction as those illustrated in FIG. 10 a. A FAIMS device 600 includesan outer electrode 404 having a part thereof cut away to enable aprotruding part 602 of the inner electrode 402 to extend into theinsulating material 410. Enough of the outer electrode 404 is cut awayto leave a wide enough physical space between the electrodes so as toprevent electrical discharge between the inner electrode 402 and theouter electrode 404. The shape of the protruding part 602 is optionallyvaried. The protruding part 602 of the inner electrode 402 forms anapproximately gas tight seal with the electrically insulating material410 to form a physical barrier which forces the gas flow, represented inthe figure by a series of closed headed arrows, around one side of theinner electrode 402. Gas entering the FAIMS device 600 through the gasinlet 412 is forced to flow in a direction toward the ion inlet 406.Near the ion inlet 406, the gas flow splits with a portion of the gastravelling out toward the electrospray needle 414, forming thedesolvation gas flow 418. The other portion, the carrier gas flow 420,transports ions through the FAIMS analyzer region 422, around the innerelectrode 402, and to the ion outlet 408.

The blockage of flow by the modification of the inner and outerelectrodes 402 and 404, respectively, results in changes in the electricfields near the modified region, causing suboptimal conditions fortransmission of ions. Therefore, the blockage is advantageously locatedin a region away from the ion path through the FAIMS device 600 so thatthe changes in the electric fields caused by the protruding part 602induce a minimal effect upon the electric fields that ions experienceduring their transit from the ion inlet 406 to the ion outlet 408.

The presence of the protruding part 602 results in increased carrier gasflow velocities through the analyzer region 422, with a concomitantincrease of an intensity of an ion stream exiting the FAIMS device atthe outlet 408. In particular, ion loss due to diffusion of ions into aregion of the FAIMS device, which is occupied essentially with extragas, is approximately minimized. Advantageously, FAIMS device 600,although more elaborate and intricate in its construction than the FAIMSdevice 400 shown at FIG. 10 a, supports analysis of an ion beam havinginitially a low ion concentration.

Numerous other embodiments may be envisaged without departing from thespirit and scope of the invention.

1. An apparatus for separating ions, comprising: a high field asymmetricwaveform ion mobility spectrometer comprising an analyzer region definedby a space between an inner electrode and an outer electrode, the outerelectrode defining an ion inlet for introducing ions into a firstportion of the analyzer region and an ion outlet for extracting ionsfrom a second portion of the analyzer region; Characterized in that: agas-directing conduit is provided through at least a portion of one ofthe inner electrode and the outer electrode, the gas-directing conduithaving an opening at a first end thereof for supporting fluidcommunication between the gas-directing conduit and the first portion ofthe analyzer region, the gas-directing conduit being adapted at a secondend thereof opposite the first end for supporting fluid communicationbetween a gas source and the gas-directing conduit.
 2. An apparatusaccording to claim 1, wherein the gas-directing conduit comprises achannel defined within the at least a portion of the inner electrode. 3.An apparatus according to claim 2, wherein an inner surface of thechannel is defined by a portion of the inner electrode.
 4. An apparatusaccording to claim 2, wherein the channel is a bore hole through the atleast a portion of the inner electrode.
 5. An apparatus according toclaim 1, wherein the gas-directing conduit comprises a tube supportedwithin a volume of space defined by an inner surface of the innerelectrode.
 6. An apparatus according to claim 5, wherein a free lengthof the tube is adjustable for varying an angle of introduction of a flowof gas into the analyzer region.
 7. An apparatus according to claim 1,comprising an ionization source in communication with the ion inlet. 8.An apparatus according to claim 7, wherein the ionization source is anelectrospray ionization source.
 9. An apparatus according to claim 1,wherein the opening at the first end of the gas-directing conduit isdisposed within a portion of an outer surface of the inner electrodefacing the ion inlet.
 10. An apparatus according to claim 1, wherein theopening at the first end of the gas-directing conduit is adapted toprovide a flow of a gas out of the gas-directing conduit and along apredetermined direction within the analyzer region.
 11. An apparatusaccording to claim 1 wherein, during use, at least a portion of a gasflow through the gas-directing conduit passes out of the analyzer regionvia the ion inlet.
 12. An apparatus according to claim 1, wherein theinner electrode and the outer electrode comprise generally cylindricalcoaxially aligned electrodes defining a generally annular spacetherebetween, the annular space forming the analyzer region.
 13. Anapparatus according to claim 12, wherein the analyzer region is ananalyzer region of a side-to-side FAIMS apparatus.
 14. An apparatusaccording to claim 13, comprising a barrier member disposed within aportion of the analyzer region for substantially preventing a flow of agas through the portion of the analyzer region.
 15. An apparatusaccording to claim 12, wherein the analyzer region is an analyzer regionof a cylindrical domed-FAIMS apparatus.
 16. An apparatus according toclaim 1, wherein the gas-directing conduit comprises a channel definedwithin the at least a portion of the outer electrode.
 17. An apparatusaccording to claim 16, wherein the analyzer region is an analyzer regionof a side-to-side FAIMS apparatus.
 18. An apparatus according to claim16, wherein the outer electrode has a length and an inner surface thatis curved in a direction transverse to the length, and wherein theopening at the first end of the gas-directing conduit iscircumferentially spaced-apart from the ion inlet.
 19. An apparatusaccording to claim 18, comprising a barrier member disposed within aportion of the analyzer region for substantially preventing a flow of agas through the portion of the analyzer region.
 20. An apparatusaccording to claim 16, wherein the outer electrode has a length and aninner surface that is curved in a direction transverse to the length,and wherein the opening at the first end of the gas-directing conduit islongitudinally spaced-apart from the ion inlet.
 21. An apparatusaccording to claim 16, wherein the opening at the first end of thegas-directing conduit is disposed within a side-wall portion of the ioninlet within the outer electrode.
 22. An apparatus according to claim16, wherein the opening at the first end of the gas-directing conduit isadapted to direct a first portion of a flow of a gas inwardly toward theanalyzer region, and to direct a second portion of the flow of a gasoutwardly away from the analyzer region.
 23. An apparatus for separatingions, comprising: an inner electrode having a length and an outersurface that is curved in a direction transverse to the length, theinner electrode comprising a gas outlet within the curved outer surfaceand at least a gas inlet, the gas outlet being in fluid communicationwith the at least a gas inlet via an interior portion of the innerelectrode, for introducing a flow of a gas provided through the at leasta gas inlet into the analyzer region; an outer electrode having a lengthand a curved inner surface, the outer electrode being approximatelycoaxially aligned with the inner electrode, a portion of the length ofthe outer electrode overlapping a portion of the length of the innerelectrode and forming an analyzer region therebetween, the outerelectrode comprising an ion inlet within a first portion of the curvedinner surface for introducing ions from an ionization source into theanalyzer region and an ion outlet within a second portion of the curvedinner surface for extracting ions from the analyzer region; and, anelectrical controller for applying an asymmetric waveform voltage to atleast one of the inner electrode and outer electrode and for applying adirect current compensation voltage to at least one of the innerelectrode and outer electrode.
 24. An apparatus according to claim 23,wherein the gas outlet within the curved outer surface of the innerelectrode is disposed opposite the ion inlet within the first portion ofthe curved inner surface of the outer electrode.
 25. An apparatusaccording to claim 23, wherein the gas outlet is in fluid communicationwith the at least a gas inlet via a gas-directing conduit disposedwithin the interior portion of the inner electrode.
 26. An apparatusaccording to claim 25, wherein the gas-directing conduit comprises achannel defined within the interior portion of the inner electrode. 27.An apparatus according to claim 26, wherein an inner surface of thechannel is defined by a portion of the inner electrode.
 28. An apparatusaccording to claim 26, wherein the channel is a bore hole through the atleast a portion of the inner electrode.
 29. An apparatus according toclaim 25, wherein the gas-directing conduit comprises a tube supportedwithin the interior portion of the inner electrode.
 30. An apparatusaccording to claim 24, comprising an ionization source in communicationwith the ion inlet.
 31. An apparatus according to claim 30, wherein theionization source is an electrospray ionization source.
 32. An apparatusaccording to claim 23, wherein the analyzer region is an analyzer regionof a side-to-side FAIMS apparatus.
 33. An apparatus according to claim32, comprising a barrier member disposed within a portion of theanalyzer region for substantially preventing a flow of a gas through theportion of the analyzer region.
 34. A method for separating ions,comprising the steps of: providing a FAIMS analyzer region defined by aspace between inner and outer spaced apart electrodes; producing ions atan ionization source that is in fluid communication with the analyzerregion via an ion inlet within the outer electrode; introducing the ionsproduced at an ionization source into the FAIMS analyzer region via theion inlet within the outer electrode; providing a flow of a gas into theanalyzer region through at least a first portion of the inner electrodesuch that a first portion of the flow of a gas flows out of the analyzerregion through the ion inlet.
 35. A method according to claim 34,wherein the first portion of the flow of a gas flows counter-current toa direction that ions are traveling in the vicinity of the ion inlet.36. A method according to any one of claims 34, comprising a step ofadjusting a rate of the flow of a gas, such that the first portion ofthe flow of a gas substantially desolvates the ions introduced into theFAIMS analyzer region via the ion inlet within the outer electrode. 37.A method according to claim 35, wherein a second portion of the flow ofa gas flows through the FAIMS analyzer region toward the ion outlet inthe outer electrode.
 38. A method according to claim 34, wherein thestep of providing a flow of a gas includes a step of directing the flowof a gas in a direction that is approximately toward the ion inletwithin the outer electrode.
 39. A method according to claim 34,comprising a step of varying an orientation of the at least a firstportion of the inner electrode relative to the ion inlet within theouter electrode.
 40. A method according to claim 37, comprising a stepof entraining the desolvated ions within the second portion of the flowof a gas.
 41. A method according to claim 34, comprising a step ofapplying an asymmetric waveform voltage to at least one of the innerelectrode and the outer electrode and applying a direct currentcompensation voltage to at least one of the inner electrode and theouter electrode.
 42. A method according to claim 34, wherein the FAIMSanalyzer region is an analyzer region of a side-to-side FAIMS.
 43. Anapparatus according to claim 7, wherein the opening at the first end ofthe gas-directing conduit is disposed within a portion of an outersurface of the inner electrode facing the ion inlet.
 44. An apparatusaccording to claim 16, wherein the analyzer region is an analyzer regionof a cylindrical domed-FAIMS apparatus.
 45. An apparatus according toclaim 21, wherein the opening at the first end of the gas-directingconduit is adapted to direct a first portion of a flow of a gas inwardlytoward the analyzer region, and to direct a second portion of the flowof a gas outwardly away from the analyzer region.
 46. An apparatusaccording to claim 23, wherein the analyzer region is an analyzer regionof a cylindrical domed-FAIMS apparatus.